1. Field of the Invention
The invention relates to a projecting device, and more specifically, to a projecting device for displaying electrical images.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a perspective view of a prior art projecting device 10 of an LCD projector. The projecting device 10 comprises a light source 12 that produces a white light beam, a uniform illumination optical device 14 installed in front of the light source 12 for converging the white light emitted from the light source 12 into a uniformly distributed rectangular light beam, a light separating device 16 for separating the rectangular light beam into red, green and blue color input light beams, a dichroic prism 18 having three input sides and an output side for synthesizing the three input light beams into an output light beam, three modulating panels 20 each formed by a monochrome liquid crystal panel and separately installed in front of the three input sides of the dichroic prism 18 for modulating the three input light beams, three focusing lenses 17, 19 and 21 separately installed in front of the three modulating panels 20 for focusing the three input light beams from the light separating device 16 onto the three modulating panels 20, and a projecting lens 22 installed in front of the output side of the dichroic prism 18 for projecting the synthesized output light beam from the dichroic prism 18 onto a screen 24. Each of the modulating panels 20 is formed by a transparent monochrome liquid crystal panel for displaying a monochrome image. The dichroic prism 18 synthesizes the three monochrome images to form the output color image.
The light separating device 16 comprises a first dichroic mirror 26 for separating the red light from the rectangular light beam, a reflecting mirror 27 for reflecting the red light from the first dichroic mirror 26 onto the focusing lens 17, a second dichroic mirror 28 for separating light reflected from the first dichroic mirror 26 by reflecting blue light to the focusing lens 19, and two optical lenses 30 and two reflecting mirrors 32 for passing and reflecting green light to the focusing lens 21. FIG. 1 clearly shows that the distance traveled by the green light is much longer than that of the red and blue lights. Since traveling distances affect light intensities, the two optical lenses 30 installed in front of the two reflecting mirrors 32 are essential to converge the green light so as to compensate for the loss of its light intensity.
However, the installation of the two optical lenses 30 makes a structure of the light separating device 16 complicated and costly. It is therefore an important objective to provide a projecting device with a simple structure that can solve the problem of unequal traveling distances for the three color beams.